Dr Fausto Martelli

Polyamorphism in water

Water exhibits polyamorphism, i.e., it exists in more than one amorphous state. The most common
forms of glassy water are the low-density amorphous (LDA) and the high-density amorphous
(HDA) ices. LDA, the most abundant form of ice in the Universe, transforms into HDA upon
isothermal compression. We model the amorphisation of liquid water into LDA and the
transformation of LDA into HDA under isothermal compression with classical molecular dynamics
simulations. We analyse the molecular structures with a recently introduced scalar order metric [1]
to measure short and intermediate range order, and we investigate the large-scale structure by the
extent to which long range density fluctuations are suppressed.
Our results indicate that the structural properties of LDA differ from those of liquid water, and that
LDA and HDA are indeed amorphous, i.e., they lack polydispersed ice domains. Interestingly, HDA
contains a small number of domains that are reminiscent of the unit cell of ice IV, although the
hydrogen-bond network (HBN) of these domains differs from the HBN of ice IV [2]. Both HDA
and LDA are characterized by an anomalous suppression of large-scale density fluctuations [3]. By
contrast, in correspondence with the nonequilibrium phase transitions to HDA, an extensive
rearrangement of the hydrogen bond network introduces strong long range density fluctuations. Our
investigations challenge the largely accepted “frozen-liquid” picture, which views glasses as
structurally arrested liquids. Beyond implications for water, our findings also enrich our
understanding of pressure-induced structural transformations in glasses.